Our society relies on computer systems in innumerable ways. Computer systems, which employ processors and other integrated circuits, control devices in our homes, in our business offices, in our manufacturing facilities, in our automobiles, and even in outer space. One can find integrated circuits in such devices as desktop and laptop computers, mainframe computing systems, and in portable devices, such as mobile telephones and palm-held computers. Many if not most of the applications are demanding improved performance from processors and other integrated circuits. Electronics designers have responded to this demand by employing various techniques and design methods to increase performance of newer integrated circuits.
Designers generally increase performance of integrated circuits by increasing the operating frequencies and by increasing the number of components, such as transistors, in the circuits. To keep the circuit sizes manageable, designers have reduced or scaled down the size of the circuit components so that larger numbers of devices fit within smaller per unit areas. Today it is not uncommon to find advanced computer system chips that contain millions, even billions, of transistors. This increased density, however, has created numerous problems. One problem is heat. Since individual electronic components, such as transistors, each generate minute quantities of heat when operating, increased numbers of such devices in the newer circuits naturally lead to increased quantities of heat. Another problem is power consumption. Again, since each electronic circuit component consumes a minute amount of power while operating, circuits with increased numbers of such circuit components generally consume larger quantities of power.
Other problems seen in newer electronic devices are those associated with scaling. While smaller circuit component dimensions have generally resulted in faster response times for many devices, such as transistor gate delays, the components often have problems related to their reduced sizes. For example, as integrated circuit die areas have decreased, transistors have suffered from problems of quantum-mechanical tunneling of carriers through thin gate oxide, both from drain to source and from drain to body. Even when faced with these problems integrated circuit designers are continually pressured to increase performance of the circuits while reducing power consumption.
As mentioned, designers have increased performance by continually scaling the circuits using smaller and smaller technologies, such as 90 nm and 65 nm technologies. They have also increased performance by increasing the clock speeds. They have reduced latencies by reducing the physical channel length of the circuit elements, reducing the voltage supplies for the elements, and reducing the threshold voltages of transistors. However, reduced threshold voltages and reduced channel lengths of transistors have resulted in higher subthreshold leakage currents. Accordingly, subthreshold leakage power, increased power consumption, and increased heat dissipation have rapidly become formidable challenges for integrated circuit designers. Moreover, with the increased use of portable electronic systems, reducing power consumption has become a paramount design concern. Power dissipation reduces battery life, decreases system performance, reduces system reliability, and increases system packaging costs.
Buffers used to manage signal delay and signal integrity problems for long on-chip buses are sources of significant quantities of leakage power. Buffers or inverters often contribute around half of total device width on chips. Additionally, inserting on-chip buffers along global lines results in higher dynamic power consumption, even when data signals remain unchanged. Reducing supply voltage results in significantly lower power dissipation, but such reduced voltage significantly impacts delay. In constant-throughput applications the performance loss associated with low voltage supply operation may be recovered by increasing pipelining or parallelism, but such techniques increase circuit latencies. What are needed are new leakage power saving schemes for such circuits as inverters, buffers, repeaters, and drivers that eliminate unnecessary dynamic power consumption, yet do not sacrifice circuit throughputs or latencies.